Validation of Checkbox list in Template form(not Reactive forms) Angular 2

How can I do the validation of checking that at least 1 checkbox is selected from a checkbox list in Angular 2 template forms?

HTML code.

   <div class="form-control" style="max-height: 300px; overflow: auto">
        <div *ngFor="let item of items" class="checkbox">
            <label for="{{item.id}}">
                <input type="checkbox" value="{{item.id}}" required id="{{item.id}}" [(ngModel)]="item.isSelected" />
                <span class="text-body">{{item.name}}</span>
            </label>
        </div>
   </div>
   <div *ngIf="!isAtleastOneItemSelected()" class="alert alert-error">
        Select at least one item
   </div>

Component Code.

public isAtleastOneItemSelected() {
    const selectedItems = this.items.filter((item) => item.isSelected);
    if (selectedItems.length > 0) {
        return true;
    } else {
        return false;
    }
}

I have already checked the Reactive form way of doing this here. So, wanted to check how we can do this in Template Forms. Using the code that I have pasted above, things are working fine but on page load the error message is showing up as there is no check for dirty or touched there. I was stuck with this issue.

Any help on this would be appreciated.

Thanks.